Two-way telephone trunk circuit



March 20, 1951 GRIMES TWO-WAY TELEPHONE TRUNK CIRQUIT Filed June '7, 1947 TO LINEFINDER BANKS 5:050 xzamh mmzho Oh INVENTOR. HARLAN E GRIMES ATTOR NEY Patented Mar. 20, 195 1 TWO-WAY TELEPHONE TRUNK CIRCUIT Harlan E. Grimes, Bromley, Ludlow Station, Ky, assignor to Automatic Electric Laboratories, Inc., Chicago, 111., a corporation of Delaware Application June 7, 1947, Serial No. 753,210

8' Claims. 1

This invention relates to telephone systems in general, but is concerned more particularly with .automatic telephonesystems in which both main automatic exchange and private automatic branch exchanges are employed; and the broad general object, briefly stated, may be said to consist of. the provision of a novel and improved circuit arrangement for a two-way trunk connecting the branch and main exchanges.

Private branch exchanges in systems of the above type maybe arranged with the. subscribers lines accessible via automatic. switches controlled by an attendant, or the subscribers lines may be multipled to manual switchboard jacks at the attendants switchboard, in which case incoming calls from the main exchange are to be completed directly by means of cord circuits.

resistance trunk-loops "and heretofore a compensating resistance was required at the trunk line circuit equipment at the main exchange to obtain satisfactory operation;

In order to overcome'the above objections, one

"of the features,v of the invention is to supply the aforementioned series relay with a second winding having a, considerably higher resistance in relation to the first winding and therefore more turns, than that of the single winding formerly used. The use of this higher resistance winding allows for a stiiier and more satisfactory adjustment of the series relay, thereby overcoming its tendency to remain operated after its operating circuithas been opened. This higher resistance winding with its 'largernumber of turns; afiords 2 erate completely and thereby closing another circuit placing its second or low resistance winding in parallel with its first Winding. Therefore these two windings in multiple reduce the series trunk loop resistance to a minimum value satisfactory for dialing impulses to be sent over the loop and for voice transmission, but developes sufficient ampere-turns to maintain the relay completely operated. The effect of the high resistance winding shuntedacross the low resistance Winding acts as a holding shunt for the relay and during the open period of a series of dialed, impulses, produces a countereelectrom'otive-force suflicient to keep the relay operated, thereby maintaining the trunk loop resistanceat the minimum value so as not to distort the-dialed impulses.

Other objects and features of' this invention are concerned mainly with circuit details which will be explained fully hereinafter with reference to the accompanying drawing, which shoWS by means of the usual circuit! diagrams the essential features of theinvention.

The drawing illustrates a private automatic branch exchange connected to a main automatic exchange by a trunk line consisting of conductors ill and It. To the right of the trunk line are illustrated certain portions of the main automatic exchange considered necessary in describing this invention with the trunk conductors ljfl and II terminating on standard line equipment, this case a combined line and cut-01f relay I60, operation of which is Well known in the art. Further the trunk extends to the right on the drawing where it reverses its polarity in respect to the line relay I60 and extends into two branches. Directly to the right hand side Of the drawing one branch of the trunk. terminates on a set of bank contacts 26 and 2''! of1a non numerical switch and in other multiply-con nected' sets which are accessible to other nonnumerical switches in the main exchange. The non-numerical switch, in this instance, is a linefinder, the exact type being considered unessena higher ampere-turn value for the restricted 45 tial to this invention. as its Op 0 is W ll *amountof' current in the trunk loop and the known in the art. This branch is for use in ex-' circuit-is so" arranged that this winding is retending outgoing calls from the private autoquired' 'only tooperate one set of preliminary matic branch exchange, to the main exchange. make contact springs,' which in turn conditions The other branch, which is used for extending .itne' trunhcii'cuit for seizure by-the selector with- 50 calls from the main exchange to the branch excut the need" or a compensating resistance at change, terminates on a set of bank contacts .210, "thetrunklineequipment; After seizure of the 2| and 22' of a connector switch and in other trunk by the selector, su'iii'cient current flows multiply-connected sets which are accessible ii through the loop to allow the higher resistance to other connectors. of. the" main exchange,

To the left on the drawi'ngrthe trunk line com usin of: the trunk; circuit series relay to opand 25. then energize and connect the line and control 24, and 25 in the well known manner. is now closed for energizing the series relay Mil as follows: ground through the upper winding of the combined line and cutoff relay I69, break contacts I64, +L conductor It, selector bank I contact 23, wiper of selector switch, the calling ductors it and H terminate on a switchboard jack Iii), which is used mostly for answering main to branch exchange calls, although under certain circumstances it may be used by the attendant operator at whose switchboard position it appears, for the purpose of extending special calls from the branch exchange to the main exchange. 'Ihe switchboard jack H is m ltinled in the well known manner to other switchboard positions by conductors 30, 3|, 32, 33, and 3d. The trunk line also terminates, at the branch exchange, in a set of bank contacts 23. 2 and 25, which with other multiply-connected sets of contacts, are accessible to well known selector switches in the branch exchange. Associated with the trunk line at the branch exchange there is also provided a trunk circuit, which consists of five relays and proper connecting circuits, which will be full described hereinafter.

As previously indicated, the exact type of switches employed is immaterial to the invention. but it may be assumed that the selectors and connectors involved are two-motion switches v of the well known Strowger type, generally similar, for example, to those shown in U. S. Patent 1,667,915, issued to J. Wicks on May 1, 1928, and

portion of a cord c rcuit which is described in Karl L. Burgners co-pending a lication Serial No. 646.463, filed February 8, 1946, now Patent 'No; 2,532.889, issued December 5 1950.

This cord circuit is one of a number of similar cord circui s which are located at the attendant operators pos tion at the branch exchange, and is used for the purpose of connecting t e branch exchange lines with the trunk c rcuit illustrated, as will be pointed out in detail hereinafter.

The drawing having been described, it will now 'be'assumed that a subscriber in the private automatic branch exchange desres to converse with fa subscriber in the main automatic exchange. will further be assumed that when a calling sub- A scribzr at the private automatic branch exchange removes his receiver and dials in the well known manner the main exchange digit in the number j of the desired subscriber, a well known Strowger .vertical and rotary selector switch seizes the first 'idle trunk extending to the main exchange.

" Assuming that the trunk line shown on the drawing is the first idle trunk line accessible to the selector switch. the w pers of the selector will then stop on the selector bank contacts 28, 2 3, The switching relay of the selector will wipers of the selector to the bank contacts 23, A circuit subscriber's loop, wiper of the selector switch,

selector bank contact 24, high resistance or upper Qwinding of series relay Ht, -L conductor H,

back contact H53, lower and middle windings in jseries of relay I69, to negative battery. Relay H0 energizes sufficiently over the above traced circuit to close' its as or preliminary contacts I I I, but does not have sufficient power, due to the restriction of the three windings of relay 166 in series, to operate its other contacts and thereby merely closes the operating circuit to slow-to-release relay I25. lelay I'm, upon operating, at make contacts E22 places ground on the trunk control conductor l2 and bank contac u 25, thus furnish'ng a holding circuit to the selector and other switches operated by the calling subscriber in extending the call to the trunk circuit and making this seized trunk busy to all other calls. Relay i2 also by way of back contact i'H lights a busy lamp :89, one of which is associated with each switchboard jack, like H0, in each appearance of the trunk on the various switchboard positions and the other busy lamps are also energized over conductor 35. Relay iZfi further, at back contacts 25 removes relay 55 from the +L conductor ii. and at contacts i23 closes the operating circuit to slow-to-release relay I39. Relay ESE, upon operating, at make contacts I32 places another holding ground on control conductor i2 and at make contacts 23! prepares a holding circuit for relay M0.

Upon the seizure of the trunk by the selector controlled by the calling subscriber and at the same time as the part al energization of the series relay MD, the combined line and cutoff relay IEE partially energizes over the previously traced circuit and closes its at or preliminary make contacts lfil, but is not energized sufiiciently. due to the high resistance winding of relay Hfi being in the trunk loop, to operate its other contacts. Upon the closure of :0 contacts H5! ground at back contacts 152 is extended over the finder start lead I! to the linefinder control relays, which in the well known manner causes the linefinder to extend the calling trunk to a free selector in the main exchange.

The selector in the main exchange, assumed to be of the Strowger type, upon seizure now extends ground back over its control wiper, bank contact 28, to the control conductor !4 which completely operates relay I by way of its middle winding. Relay [50, upon completely operatng, at back contacts H52 removes the starting ground from conductor ii and at back convtacts I63 and B64 removes its lower and upper windings from the trunk conductors l and 59,

respectively. thereby clearing the trunk of appendages. The line relay of the seized selector of the main exchange is connected across the to a value sufficient to effect complete operation of the series relay H0,- Relay l|0, upon complete operation, at make contacts 1 i2 connects 'ltfi low resistance or lower winding in multiple with its upper wind ng, thereby reducing (the trunk loop resistance and conversely increasing the current to a value consideredpractical for dialing and voice transmission.

The trunk circuit, the linefinder and the seized selector are now in condition to extend the connection, dial tone being returned by the seized selector back to the callingsubscriber; The calling subscriber now manipulates hiscaling device in accordance with the remainder of the directory telephone number of the desired subscriber in the "change", with the re ult that the "desired im is completed the 'des'ired sub? s bell is rung by the connector switch in l 'n' 'r. Relay H will not respond to g ir'npu s es due to theshuntln'g eliect of its high resistance winding on its low resistance iv" riding, thus maintaining a low resistance trunk and p eventing distortion of the dialed rm: pulses; When the called subscriber ariswers, the ringing current is cut dif by the connector in the usual manner and the talking circuit is completed through the by p'ass condenser I90 can: fie ed across the windings of relay I I0, where'- u" on the two subscribers converse in the usual ihanner. V

Wherjr theconyersation is completed; the two subs 'bers replace their rece vers. When the e1yer for the calling s'uhs'criber at the branch exc ange is re laced, the circuit or relay n0 of the trunk circuit and the iihe 'eiay or the coniiector ill the main exc ange is Opened. Relay "0, upon restoring, at contacts I II opens the circuit to 'slow-torelease relay I20, which thereupon'shortly restores and at contacts I22 removes its ground from thecontr'ol conductor I2, at contacts I23 opens the operating circuit to relay I30 and at'contact's I2'I re-conneots'relay I50 to the trunk eh'diieto'r I0. Relay I30, after a slight delay, restores and fat contacts I32 further reve'e ground from control "conductor I2. jqrloilnd now removed from control conductor I2 allows the release of the switches which were bp'erated by the calling subscriber and also eX ting'ui'shes the busy'la'mp I80. v g Shortly after the restoring of the line relay 'of the connector switch in the mainexchange ,ida'ii'sed by the calling subscriber hanging up his receiver, ground is rino'vedfrb'm the control conn tter I4 thereby restoring the nominee line 'andhutofi relay I60 to normal. The line relay of the connector will not be held energized by the reconnection of relay I50 to the trunk conductor fIo dueto the high resistance-er rela I53. The now free and ready to receive another 'Call.

fi-Iaving described the operation of the trunk "circ uit'in connection'with 'a call from the priv'a'te branch exchange to the'ni'ain exchange, an expla nation will new be given of "a call from the "fmainfexchange to a subscriber in the private automat c hraneh exchan e, It will now be asf'sjulned that thesubscriber at the main automatic "exchange desiresto converse with a' 's ulcscrib'er in 'thepriv'ate automatic "h'hicchang'e and that he accordingly removes-his 'eceiverarfddials the h mberjerjthe branch exchange as listed-in the telephonedirect'ory. v h "Itwill'further be as'sunied that aconnector Switch in the main exchange is'eiz'esthe trunk'ter- 1Ln1 na'ls'20, 2|,and 22, responsive to the-dialing ffof the digits "of the telephone nuniberbf the private branch exchange. Theconnector switch, fupoln seizing the jbanl;*contacts20,"2i and 220i 3 the trunk line; places ground on control terminal "22, whereupon 'a circuit is closed for operating cornpletelythe combined line and "ciitdfirela'yll 60 y we of ite rriiddle winding. Rela its, upon Yoperating completely, removes itself irom the trunk conductors I0 'and I] atcontacts I64=ahd "1163 an prevents any operationoi linefinder ,hyfremoving f gr ound ijat" contacts I02; "A' further resiutor the connectorswitch "seizing the trurik banlgcontacts 20, 2I, and agreeingh the ener- "g tion'ofhi'gh resistance relay" I50 ofjthe trunk "circuit; The circuit for"energiiirig relay' 'flitl iiiay be traced as follows; from battery superimposed on the ringing circuit, through the winding of the connector ring cutofi relay, over the connector switch wipers to bank term.na1 20, -L conductor It, +'L" trunk conductor i6, back contacts I2I-, winding high resistance relay I53 to ground. Relay I56 is energized either during the ringing period of the connector switch, or during the silent period. Relay I50; upon operating, at con tac'ts I5I closes an operating circuit to slow-torelease relay I30. Relay I, upon operating, at contacts I32 connects ground to control con-' ductor I2, thereby making the trunk busy to the selector switches of the private branch exchange which have access thereto and lighting the busy lamp I80, and 'atcontacts I3I prepares a locking circuit to relay I40. Relay I50 is of such high resistance that the ring cutoff relay or" the connector wi11-not operate. When ringing current is applied to the trunk circuit at the connectar switch, relay I140 operates over the previously traced circuit for relay I56, through ringing condenser I9I, lower winding reay I40, back contacts I43, trunk conductor I I, trunk bank terminal 2| to ground in the connector.

Relay I40, upon operating, xf or preliminary make contacts MI locks throughiits upper winding to ground at back contacts I22, at back contacts M3 :opens its operating circuit and at make contacts I 32 c-lcses a circuit for lighting the line lamp I8I which indicates to the operator at the,

private branch exchange that a subscriber at the main exchange desires connection with a suxiiscri'l'rer in the private branch exchange.

sponsive to-the plugginginofthe plugP into the jack I10, a circuit is closed for energizing relay H6 in series with the relay "I0 of the cord circuit. The closing of the trunk loop by the connection of relay "I0 across it operates the ring cuto'fi relay in the connector which in turn switches, the trunk ;throug-l-1 to the connector back-bridge relay. Relays "I0 and H0 operate 'over the following-circuit; ground through one winding of the connector back-bridge relay, conhector wiper, bank terminal 2|, L=conductor "I5, -'L "trunk conductor -I I, upper Winding -relay IIEl, ring terminal in of the-jack I10, ring terminal of the plug P, winding relay 16, back 'cont'actshfi'l tip terminal of i the plug P", tip terminal I12 of -the jack-I10, -L trunkconductor I0, L-conductor I6, bank terminal 20, connector Wiper, to "negative battery through the f oth-er windingof the connector back-bridge =re- "la'yi -Relay H0, upon operating completely due to the large value ofcurrentinthe loop being ied from the connector back-bridge relay, at;mal e contacts I'I'I' closes the" circuit to relay "I N and at make "contacts 'II-2 placesits low resistance winding in parallel "with itshigh-resista-nc'e winding. Relay I253, 'upon "operating, at back contacts I22 opens'the holding circuit'to relay I 40,

"atmake contacts I22 places a'further holding ground on control conductor" I2, atbackc'ontacts contacts I23 closes a holding circuit 'for' relay I30; trunk circuit is now 'incondition forco'nversa- Relays I40 and I restore to'normal. The

thin.

Relay 10 of the cord circuit, upon-operating,

closes a circuit for lighting iai supervisory *lamp 75 at 'coht'actsu'llgsi; i

The operator now throws a key, not shown, thereby connecting her headset to the conductors of the cord circuit. The operator now converses with the calling subscriber and ascertains from him the number of the desired substation. Upon learning that the calling subscriber desires to converse with one of the subscribers at the private branch exchange, the operator checks to determine if the busy lamp I80 associated with the called subscribers jack is lighted. If the busy lamp is not lighted, indicating the line is idle, the operator inserts the plug P into the jack of the called line and rings the called party in a well known manner. When the called subscriber at the branch exchange removes his receiver, a

with the bridge across the trunk circuit which in cludes relay H1. As a result of the direct current bridge placed across the line by the answering party, the supervisory relay operates in series with the back-bridge relay of the connector switch in the main exchange. Relay 6%, upon operating, at contacts (BI removes relay "ID from the cord circuit, which in turn extinguishes the supervisory lamp indicating to the operator that the call has been answered. The calling and called subscribers may now converse with each other as desired.

When the conversation is completed, the two subscribers replace their receivers.

It will be assumed that the calling subscriber in the main exchange is the first to replace his receiver. The line relay of the connector switch in the main exchange de-energizes and prepares the circuit for releasing the connector. The connector, however, does not release because the back-bridge relay is maintained energized by the called subscriber. The switches used by the calling subscriber in the main exchange with the exception of the connector switch, release in the well known manner. The connector switch, however, maintains ground on the control trunk terminal 22, to maintain the combined line and cutoff relay ISIJ operated until the called subscriber hangs up.

When the called subscriber at the branch ex change replaces his receiver relay 63 in the cord circuit restores and at contacts SI replaces relay iii across the trunk loop. Relay Ill, upon operating, at contacts II re-lights the supervisory lamp indicating to the operator that th called subscriber has hung up and that plugs P and P may be removed from their respective jacks.

Upon the removal of the plug 1? from the jack I'Ifi the trunk loop is opened and the back-bridge relay of the connector switch in the main exchange and the series relay I I0 restore to normal. The restoring of relay III! opens the operating circuit to slow-to-release re ay I28, which in turn opens the operating circuit to slow-to-relea e rclay I38. Shortly after the restoring of relay I It, relay I26 restores and then relay I38 restores, thereby removing ground from control conductor !2 extinguishing the busy lamp I80 and removing ground from the selector bank terminal 25. Shortly after the restoring of the back-bridge relay in the connector, ground is removed from the control terminal 22 and the combined line and cutoff relay restores. The trunk is now free and ready for other calls.

The operator at the branch exchange with the aid of the cord circuit and a calling device such as a dial, not shown, is able to set up a call to the main automatic exchange by inserting the plug P into the jack IIU after which she dials the number of the desired subscriber in the main ex-{ change. The operation of the trunk circuit is the same as previously described for completing a connection from a private branch exchange subscriber to a called main exchange subscriber.

On a call from a subscriber at the private branch exchange to a toll operator or the like in the main automatic exchange means is required so that the toll operator may at will hold the trunk even though the calling subscriber has hung up his receiver. Assuming that a subscriber at th branch exchange has dialed a toll operator at the main exchange, the seizure of the trunk and extension of the trunk loop to the toll operators position is the same as previously described for a call from a subscriber in the branch eX-, change to a subscriber in the main exchange. It is assumed that the toll operator has answered the calling party and has ascertained the information required for placing, say a call to a distant exchange. If for some reason the call cannot be completed immediately the toll operator can tell the calling subscriber to hang up his receiver and that she will ring him when the call is ready. The hanging up of the receiver by the calling subscriber in this case will not release the trunk as long as the toll operator keeps her answering plug in the answering jack. Upon the opening of the loop by the hanging up of the receiver relay II!) in the trunk circuit restores and at contacts II I opens the circuit to slow-torelease relay 520.

After a short delay, relay I28 restores and at contacts I2I connects relay I52 to the trunk conductor IE3, at make contacts I22 removes one of the holding grounds from the control conductor I2, at back contacts I22 prepares the holding cirecuit 'for relay Hi] and at contacts I23 opens the operating circuit to slow-to-release relay I30. Relay I5i! now operates from negative battery through the line relay of the selector in the main exchange on which the toll operators position terminates, wiper of the selector switch, bank contact 2?, L conductor I6, +L conductor IO, contacts I2I, winding rela I58, to ground. Relay I50, upon operating, at contacts I5I closes another operating circuit to relay I30, which be.- ing slow-to-release has remained operated after the release of relay I29. The trunk circuit is now held and the ground on control conductor I2 maintains the selector and other switches operated in the branch exchange.

When the toll operator is ready to complete the wanted connection she operates her ring key which places ringing current on the trunk and relay I40 operates, as previously described, at contacts I II locks to ground at contacts I22, and at back contacts I43 removes itself from the trunk loop. The ringing current also rings the calling subscribers bell and when he answers by removing his receiver the loop is again closed and relays IIS and I2!) operate and relays I40 and I50 restore as previously described.

The release of the trunk circuit and the associated operated switches is dependent upon the toll operator. This release is efiected by the toll operator removing the answering plug from the jack associated with the calling trunk which in turn allows the release of the selector associated with the toll operator.

Having fully described and ascertained the features and aspects of the invention, what is con- ,sidered to be new and desired to have protected by Letters Patent will be pointed out in the ap,- pended claims.

What is claimed is: I

Inatemnhonesystem, a main exchange and a branch exchange, a two conductor trunk connecting said exchanges, means for seizing said t ims etseis ra ch e hanges Se s el y a i'n'g'a high resistance winding connected-inseries with one conductor of said trunk, ahl eying relay, a first setof contacts on said series relay operated in response to said seizure for operating said ibusying irelay .to busy said ,trunk at said branch exchange, finder switches in said main exchange having access to said trunk, means in said trunk responsive to said seizure for operating an idle one of said finders to connect with said trunk and to extend the same, a low res'stance' winding on said-series relay, and a second set of contacts on said series relay operated in response to said connection of said one finder to said trunk for connecting said low resistance winding in multiple with said high resistance winding to reduce the resistance included in series with said one conductor of said trunk.

' 2. In a telephone system, a main exchange and a branch exchange, a two conductor trunk connecting said exchanges, means for seizing said trunk at said branch exchange, a series relay having a high resistance Winding connected in series with one conductor of said trunk, finder switches in said main exchange having access to said trunk, means in said trunk responsive to said seizure for operating an idle one of said finders to connect with said trunk and to extend the same, a low resistance winding on said series relay, and a set of contacts on said series relay operated in response to said connection of said one finder to said trunk for connecting said low resistance winding in multiple with said high resistance winding to reduce the resistance included in seties with said one conductor of said trunk.

3, In a telephone system, a main automatic exchange and a private automatic branch exchange, a two conductor trunk connecting the main exchange and the branch exchange, said trunk terminating in multiple switch banks at the branch exchange and in an automatic switch at the main exchange, a relay having a high resistance Winding and a low resistance winding, said high resistance winding connected in series with one conductor of said trunk, said trunk seizable through said switch banks, means in said trunk operative responsive to sa d seizure for busying said trunk, other means insaid trunk also responsive to said seizure for causing said automatic switch to hunt and seize an idle path in the main exchange, and means responsive to connection of said trunk to said idle path to connect said low resistance winding in multiple with said high resistance winding.

4. In a telephone system, a main automatic exchange and a private automatic branch exchange, a two conductor trunk connecting the main exchange and the branch exchange, said trunk terminating in multiple switchboard jacks at the branch exchange and in an automatic switch at the main exchange, a relay having a high resistance winding and a low resistance winding, said high resistance winding connected in series with one of the two conductors of said trunk, means for seizing said trunk at said switchboard jacks, means i n' said trunk operative responsive to said seizure for busying said trunk, other means in said trunk also operative responsive to said seizure for the chan and t e b Initiating in mi l-tipl s branch-exchaneeandin m u.

l ance, a re a wine a eh'resistwindin and low re istance hi h wi dieezconnccted ione conductor of said trunk, Said trunk seizable through any one of said switch banks in response to an incoming call, plug and cord means for answering said incoming call at sa d switchboard jacks, and a circuit completed through said plug and cord means responsive to such answering of said call for causing said relay to operate and connect said low resistance winding in multiple with said high resistance winding, thereupon re-e ducing the loop resistance of said trunk.

6. In a telephone system, a main automatic exchange and a private automatic branch exchange, a two conductor trunk connecting the main exchange with the branch exchange, said trunk terminaing in multiple switch banks at the branch exchange and in non-numerical hunting switches at the main exchange, a relay having a plurality of sets of contacts, a high resistance winding of said relay in series with one conductor of said trunk, said winding operative upon seizure of said trunk for operating only one set of said contacts, means responsive to the seizure of said trunk at said banks for operating an idle one of said non-numerical switches to extend the connection. said windin operative in response to said extension for o erating said relay completely to operate its remaining contacts, and a low resistance winding of said relay connected in multiple with said high resistance winding in response to the operation of said last mentioned contacts.

'7. In a telephone system, a main automatic exchange and a private automatic branch exchange and a private automatic branch exchange, a two conductor trunk connecting the main exchange with the branch exchange, said trunk terminating in multiple switchboard jacks at the branch exchange and in non-numerical hunting switches at the main exchange, a relay having a plurality of sets of contacts, a high resistance winding on said relay connected in series with one conductor of said trunk, said Winding operative upon seizure of said trunk for operating only one set of said contacts, means responsive to seizure of said trunk at said jacks for operating one of said non-numerical switches to extend the connection, said winding responsive to said extension for operating said relay completely to operate its remaining contacts, and a low resistance winding on said relay connected in multiple With said high resistance winding in response to the operation of last mentioned contacts.

8. In a telephone system, a main automatic exchange and a private automatic branch exchange, a two conductor trunk connecting the main exchange and the branch exchange, a high resistance relay normally connected between one of said trunk conductors and to ground, said trunk seizable from said main exchange, means in said trunk responsive to such seizure for ma};-

lng said trunk busy at the main exchange end, said relay also energized responsive to said seizure, means responsive to operation of said relay -to-make said trunk test busy at the branch ex change end, an operators position including a cord circuit, a second relay having a high resistance winding and a low resistance winding, 'said'high resistance winding connected in series with one conductor of said trunk, circuit means responsive to connection of said cord circuit with said trunk for operating said second relay. means responsive to operation of said second relay for connecting said low resistance winding in multiple with said high resistance winding, and other 12 means also responsive to the operation of said second relayior de-energizing said first relay and for maintaining said trunk busy at the branch exchange terminals.

HARLAN E. GRIMES.

REFERENCES CITED UNITED STATES PATENTS Name Date Toomey Jan. 9, 1940 Number 

